This invention relates to a data transmission control method for communication using a data transmission network by the multicast systems for communication between one sender station and some specified receiver stations. More specifically, the present invention relates to a data transmission control method capable of reducing traffic density when data is transmitted to some specified destinations, and also relates to a station used under this method.
To give an example of a conventional data transmission control system of this type, there is a known system which collects information about the states of the individual stations by rotating a frame in a loop transmission line in a loop network system having the stations connected in a loop transmission line. In this data transmission system, a station, which is going to collect information about the state of other stations, need not send a frame to each station one by one, and therefore can collect information about other stations efficiently.
A method for managing token ring LAN (Local Area Network) by using a frame with information about a command to request that a specific process be performed at a destination system is shown in pp. 31-39 of IEEE 802.5-1989, but this method can only be used for management communication either in a point-to-point or broadcast transmission.
As a method for sending data to some destinations simultaneously, broadcast is known in LAN technology. Broadcast is a communication technology which transmits many request and response frames to all systems on subnetworks connected by internetworking devices in a whole network.
In LANs of the IEEE 802 family, there is provided means for enabling frames to be received by only stations specified by a "group address", and this scheme poses problems that traffic increases like in the above-mentioned broadcast, that it is necessary to do prior definition of groups that reduces the flexibility of communication, and that since group addresses are limited resources, before it becomes possible to use them, you have to apply to IEEE and pay a specified registration fee.
Referring to a sequentially broadcast data verification system for confirming normal arrival of broadcast data which is transmitted sequentially, as disclosed in JP-A-4-124933, a problem with the conventionally broadcast data verification system is that since the source station sequentially receives confirmation messages from the individual terminals of the destinations of all broadcast data, it is necessary to secure a great amount of line retention time in order to verify transmitted data with the destinations of the broadcast data, which imposes a great load on the source station of broadcast data and the subnetwork connected to the station. For this reason, the sequential broadcast data verification system disclosed in the above-mentioned Patent Office Journal is aimed at solving the above problem by adopting an arrangement that broadcast data is transmitted by a destination that receives the data to the next destination according to the specified sequence until it comes back to the source station.
However, the above-mentioned broadcast technique has a problem that since the frames are sent to areas to which data need not be transmitted, if some systems repeat broadcasting, the traffic rises higher by synergistic effect than in a point-to-point transmission.
Furthermore, in the above-mentioned sequential broadcast data verification system, when sending a broadcast data, the source station generates a broadcast frame having included in its destination field information about the sequence of destinations, and destination stations, as they receive the broadcast frame, sequentially transmit the broadcast frame according to the specified sequence in the destination sequence information. Then, when the broadcast frame is sent back, the source station makes a decision whether or not the broadcast frames sent out and sent back coincide with each other and thereby makes sure that the transmission of the broadcast data has been completed. Therefore, if any of the destinations was unable to receive the broadcast data by some fault or other, the broadcast data is not sent to the subsequent destinations, and the source station cannot make sure whether or not transmission has been completed, which reduces reliability. In addition, when a destination station is unable to receive broadcast data by power supply being off or the line being busy or the like, there is a possibility that the station is ready to receive by retry after passage of some time. In the above-mentioned sequentially broadcast data verification system, however, since a transfer sequence is decided when the source station broadcasts data, the system is lacking in flexibility such that the transfer sequence be decided according to the condition of the destinations, for example.